1. Technical Field
The invention relates to a method and an apparatus for determining the storage state of an ammonia-storing SCR catalyst.
2. Discussion
The main sources of nitrogen oxide emissions (NOx) in the industrialized states are traffic, fossil-fired power stations and industrial installations. While the power-station and industrial emissions are being increasingly reduced, the proportion accounted for by traffic is coming increasingly to the forefront.
The NOx emissions of petrol-operated spark-ignition engines can be drastically reduced by operating at xcex=1 and by post-engine emission control by means of a three-way catalyst. Owing to the principle concerned, this possibility does not exist in the case of a mixture-regulated diesel engine operated with a mixture leaner than stoichiometric. On account of the high oxygen content in the exhaust gas, so far it has not been possible to produce a catalyst which can reduce the NOx emissions without the addition of reducing agents, generally hydrocarbons or ammonia-forming compounds.
To remove the nitrogen from power station emissions, SCR processes (selective catalytic reaction processes)xe2x80x94 as described for example in DE 245888 xe2x80x94are used in order to convert nitrogen oxides selectively into water and nitrogen by adding the reducing agent ammonia (NH3). Such control has proven suitable given the slow changes over time in the volumetric flow of exhaust gas and NOx concentration occurring in the power station sector.
The complicated processes taking place in the SCR process can be described in a simplified form by equations (1) and (2)
4NO+O2+4NH3xe2x86x924N2+6H2Oxe2x80x83xe2x80x83(1) 
NO2+NO+2NH3xe2x86x922N2+3H2Oxe2x80x83xe2x80x83(2) 
Such an SCR process can also be used in a modified form for the removal of nitrogen from diesel-engine exhaust gases. For use in a diesel-operated motor vehicle, in particular a commercial vehicle, numerous processes for the reduction of nitrogen oxides in exhaust gases by controlled NH3 addition are therefore described, for example in; (1) Lepperhoff G., Schommers J.: Verhalten von SCR-Katalysatoren im dieselmotorischen Abgas [Behaviour of SCR catalysts in diesel-engine exhaust gas]. MTZ 49, (1988), 17-21; (2)
Hxc3xcthwohl G., Li Q., Lepperhoff G.: Untersuchung der NOx-Reduzierung im Abgas von Dieselmotoren durch SCR-Katalysatoren [Investigation of NOx reduction in the exhaust gas of diesel engines by SCR catalysts]. MTZ 54. (1992), 310-315; and (3) Maurer B., Jacob E., Weisweiler, W.: Modellgasuntersuchungen mit NH3 und Harnstoff als Reduzierungsmittel fxc3xcr die katalytische NOx-Reduktion [Model gas investigations with NH3 and urea as reducing agents for catalytic NOx reduction]. MTZ 60 (1999), 398-405.
The unknown NH3 charging state (filling level) of the SCR catalyst in non-steady-state operation proves to be problematical. It is characterized by adsorption and desorption, which occur at different catalyst temperatures. Furthermore, the mass throughput or space velocity of the exhaust-gas flow and the content of NOx or NH3 in the exhaust gas also affect the charging state. The ageing of the catalyst is also a factor which must not be ignored.
FIG. 1 schematically shows a detail from a cross section of a typical SCR catalyst 10. Here, the porous catalyst material 12 is permeated by channels 14 through which the exhaust gas flows, also known as xe2x80x98cellsxe2x80x99. The cell density of such materials may be up to several hundred cells per square inch. With this structure, the porous catalyst material 12 has three tasks: primarily, the constituents of the catalyst permit the desired reaction processes within the available temperature range, furthermore the extruded material provides a mechanically sturdy unit which does not require any additional support components, and finally it permits the adsorption and desorption of NH3.
In the case of the supported catalyst 20 shown in FIG. 2, the actual catalyst material is applied as a coating 22 to a substrate 26, for example cordierite. The substrate 26 likewise has channels 24 through which the exhaust gas flows.
A schematic overall view 30 of a catalyst is represented in FIG. 3.
The exhaust gas flows in the z direction.
As FIG. 4 reveals, a typical catalyst material, shown here by way of example, consists of the semiconductor metal oxides titanium oxide (TiO2), vanadium oxide (V2O5) and tungsten oxide (WO3). These semiconductor metal oxides can change their physical properties, in particular their electrical properties such as conductivity and permittivity, with the chemical composition or by the adsorption of NH3 surface charges.
The amount of NH3 supplied to the catalyst is partly converted directly on the surface with NOx and the remainder is adsorbed in the porous catalyst layer. If more ammonia than can be converted by the reaction with NOx is supplied, adsorption of the excess ammonia occurs in a way corresponding to the profile sketched in FIG. 5a. FIG. 5a shows in case A a catalyst saturated with NH3 at the inlet of the catalytic converter. Assumed by way of example is a maximum adsorption capacity of 4 g of NH3/kg of catalyst mass. The NH3 mass not reacting with NOx can no longer continue to be adsorbed at the inlet of the catalytic converter and, in the example represented, only finds adsorption possible again after about 200 mm of the length of the catalyst. An xe2x80x98NH3 xe2x80x99 front is formed, descending over the length of the catalyst from the saturated state to 0 g/kg. If the excess supply of NH3 continues, this NH3 front moves in the direction of the outlet of the catalytic converter. In case A* represented, part of the excess NH3 is already emitted (NH3 leakage) even though the catalyst is not yet saturated over the entire length.
The adsorption capacity of the catalyst is dependent on the catalyst temperature. Case B in FIG. 5a shows the amount of adsorption over the length of the catalyst for an increased temperature. With approximately equal NH3 leakage, the integrally stored amount of NH3 in case B is significantly reduced, see also FIG. 5b. 
With a controlled addition of NH3, determination of the NH3 filling level is performed by computation and so far it has not been possible for this to be verified by measurement. To prevent NH3 breakthrough, the adsorption capacity of the catalyst must not be used up completely on account of the relatively inaccurate computation of the filling level; as a safety measure, additional storage volume must be kept in reserve, taking up additional installation space.
In the event of malfunctions, so far it has not been possible for an increased filling level to be detected. Changes in the NOx emission of the enginexe2x80x94for example due to changed ambient conditions (atmospheric humidity, air temperature), engine ageing, production variations, etc.xe2x80x94, or changes in the catalyst properties (for example ageing, reduction in the adsorption capacity) influence the mass of NH3 to be adsorbed in the catalyst and are not covered by the filling level calculation.
To ensure a correct metered amount of the reducing agent ammonia or an ammonia-forming compound, such as urea for example, the literature proposes use of one or more exhaust-gas sensors for regulating the amount of the metered agent. Thus, EP 0 554 766 A 1 presents a method which requires one or two NOx sensors. DE 41 17 143 A1 proposes a method which requires one NH3 sensor and DE 42 17 552 C1 proposes a method in which two NH3 sensors prove to be necessary. For a further method, proposed in DE 195 36 571 A1, an NH3 sensor is likewise indispensable.
All the methods mentioned employ control methods which are very complex and scarcely cover all eventualities, since, as stated above, the charging state of the SCR catalyst is dependent on very many operating parameters, which also to a great extent involve the prehistory, i.e. earlier operating states.
If it were possible to detect the charging state of the SCR catalyst by a suitable method, it would be possible to dispense with the exhaust-gas sensors mentioned above, or under certain circumstances only one NH3 sensor would have to be fitted in the exhaust pipe as a switch, in order to diagnose a malfunction. It would be possible to dispense with complex and inaccurate control strategies. In addition, the volume of the catalytic converter (and consequently the installation space required) could be reduced by that part which has to be provided in the ase of exclusively open-loop controlled systems to allow additional adsorption in the catalyst of excessively metered amounts caused by inaccurate calculation of the stored amount of NH3 and prevent harmful NH3 leakage.
In U.S. Pat. No. 5,546,004 there is a description of a sensor for determining the storage state of an SCR catalyst. This involves measuring the electrical conductivity of a material which is identical to the SCR catalyst material with regard to its physical properties. The recording of the electrical conductivity takes place within the material at a plurality of points which are at different distances from the surface of the material around which the exhaust gas flows. The measurement of the variation in electrical conductivity in dependence on the depth of the material allows conclusions to be drawn concerning the concentration of the substance adsorbed on the sensor material.
DE 196 35 977 A1 describes a method for determining the storage state of an NOx storage catalyst. This involves sensing a physical property of the catalyst material changing chemically with the NOx storing process, a measuring pickup being applied to the NOx storage material.
Alternatively, a material which is identical with regard to its physical properties may also undergo measurement directly.
The invention is based on the object of providing a method for detecting the storage state of an SCR catalyst which can be realized costeffectively and with which NH3 leakage can be avoided with a great safety margin.
According to the invention, the change in at least one physical property of the SCR catalyst material, changing with the NH3 storing process, is sensed, the measurement taking place on the SCR catalyst material itself by applying a measuring pickup to the SCR catalyst material or bringing it into direct contact with the latter and determining the storage state, for example the filling level, on the basis of these results. Alternatively, a sensor whose function-determining element consists of the same or a similar material or a material of the same physical properties (referred to hereafter as xe2x80x98substitute materialxe2x80x99) may be introduced into the exhaust-gas stream, in order to infer from the physical properties of this substitute material the storage state of the SCR catalyst. In this case, the measurement takes place on the substitute material by applying the substitute material to the measuring pickup.
In both alternatives of the method, consequently the physical properties of the SCR catalyst material, or of the substitute material itself are sensed. These methods have major advantages over indirect methods, in which measuring signals outside the SCR catalyst or the substitute material (for example NH3 breakthrough) are used to conclude their properties.
In this case, the measuring pickup is applied directly to the SCR catalyst.
In an advantageous embodiment of the invention, the sensing of the physical property takes place at a plurality of points of the SCR catalyst, so that a location-dependent determination of the storage state is possible.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from a reading of the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.